An EDM system commonly comprises a plurality of subsystems, i.e. a mechanical component or machine proper, a power-supply unit, a dielectric-supply unit and an dielectric-liquid control unit. The power supply provides a succession of controlled machining pulses across a machining gap formed between a tool electrode and a workpiece to create a corresponding series of electrical discharges through the dielectric-filled gap to remove material from the workpiece. The mechanical part or machine proper includes a column, a worktank and an electrode head or a work table which is associated with an electrode servo feed unit for controlledly advancing the tool electrode relative to the workpiece. The electrode head may also be equipped with a chip removal mechanism for reciprocating or cyclically retracting the tool electrode away from the workpiece, thereby facilitating the removal of machining chips from the machining area. The dielectric-supply unit is provided to handle a machining liquid medium and deliver it into the machining gap at a controlled rate or pressure. The electrical control unit is required to act on these subsystems, e.g. to adequately control the positioning of the tool electrode relative to the workpiece, the rate of relative electrode advancement and retraction, parameters for the machining pulses and the rate of delivery of the machining liquid into the gap and may also include a variety of safety and regulation arrangements including short-circuit protection and power shut-off circuitry.
The control unit must be associated with a gap monitoring electrical circuit unit which monitors the progress of machining in the gap by sensing the gap current, voltage, impedance and/or the high-frequency component in the gap voltage or current on a per-pulse or average basis, thereby judging the suitability of the gap size or physical conditions.
In a conventional EDM system, the gap detector unit electrically connected with the machining gap is disposed rather remotely from the gap site at which machining discharges actually take place or is designed to detect the voltage or current in the circuit at a point or points remote from the machining gap.
The EDM process is, however, characterized by the utilization of high-energy and high-frequency transient arc discharges. The resistance in the power-supply circuit is of an extremely low value, it being noted that it is inadequate to neglect the presence of stray capacitance and inductance in the power circuit lines. It should also be noted that the EDM gap constitutes a source of noise over a wide frequency band and does not necessarily yield a precise and reliable indication of the state of electrical discharges or machining in the EDM gap.
As detection and signal lines are lengthened, the detector and control circuits tend to malfunction due to the noise which accompanies gap discharges and the induction caused by the current passing through the power-supply lines.
In conventional EDM systems, it can be said, therefore, that the detection of the gap state has been achieved only incompletely, often accompanied by misjudgment, and control operations based upon inadequate detection have tended to lead to unsatisfactory results.